The present invention is concerned with the provision of cathode sputtering targets having low concentrations of mobile ion contaminants, such as sodium and other alkali metals.
In the fabrication of microelectronic integrated circuits, component inteconnections typically are formed from thin deposited metal films. A variety of procedures has been employed to deposit microcircuit metallization layers, but vacuum evaporation and cathode sputtering are the ones most commonly used at present. Cathode sputtering, the older of the two methods, is particularly suitable for depositing alloys and highly refractory metals.
Techniques for depositing materials by cathode sputtering are well known. A cathode target formed of the material to be sputtered is subjected to intense bombardment by gas ions in a low pressure environment. Atoms of the cathode material are physically dislodged by the bombardment and diffuse through the gas to deposit as a film on a substrate placed in their path. A potential of about 2 to 5 kV typically is applied between the cathode and a nearby anode, which may be used to support the substrate to be coated. The resulting electric field between the anode and cathode accelerates the ions and causes them to bombard the exposed surface of the cathode. In some types of sputtering equipment, a magnetic field is utilized in to enhance the deposition rate. Apparatus of the latter type, generally referred to as magnetron-type systems, is described in The Review of Scientific Instruments, Vol. 40, No. 5, pp. 693-697 (1969) and U.S. Pat. No. 4,166,018 to Chapin.
In modern, commercially available magnetron-type sputtering systems, the cathode target usually is a flat plate formed of the material to be sputtered. The plate is attached in some suitable manner to a carrier or support that conducts process-generated heat to a circulated liquid coolant. Target plates are formed in a variety of ways, depending on the material involved. For example, targets of palladium or nickel-chromium alloy may be vacuum cast from the molten metal. Titanium-tungsten (Ti-W) targets are produced by sintering particles of the constituent metals together in a graphite mold using a hot-pressing procedure. Zinc oxide targets can be made by pressing the powdered material into a mold of the desired configuration.
As is generally appreciated, contamination must be avoided at every stage in an integrated circuit manufacturing process if performance and yield goals are to be achieved. This is particularly true in the case of circuits built using MOS (metal-oxide-semiconductor) technologies and those designed for high voltage or high speed applications. The need to control contamination of course extends to the metal thin-films that are deposited in the metallization process. In this connection, it has been determined that certain, "mobile" ions, such as sodium, are particularly deleterious and that metal films deposited directly on silicon dioxide insulating layers should have a very low concentration of mobile ion contaminants. Problems caused by mobile ion contamination include the creation of unwanted leakage paths and short-circuiting of isolation diffusions in bipolar devices, and the degradation of low current beta in lateral PNP and NPN transistors. In addition, mobile contaminants produce shifts in the gate threshold voltages of MOS transistors and instabilities in the breakdown potentials of high voltage devices. Mobile ion contamination may, of course, result from a lack of cleanliness in the IC fabrication process. However, it cannot be eliminated or reduced to an inconsequential level unless the materials used to manufacture the circuits are of sufficient purity themselves. As already noted, metal thin-films that contact silicon dioxide insulating layers should have a very low level of mobile ion contaminants.
Sputtering targets with sufficiently low concentrations of mobile ion contaminants, sodium in particular, have not been commercially available for certain desireable metal systems. For example, gold is a preferred contact metal in some integrated circuit applications, but it does not adhere well to silicon dioxide and an intermediate bonding metal layer must be used. An alloy of titanium and tungsten (10% Ti--90% W by weight) forms excellent bonding layers with silicon dioxide and is a good barrier metal, but commercially available Ti-W sputtering targets have been found to contain excessive concentrations of sodium. Accordingly, one object of the present invention is to provide Ti-W targets having low concentrations of sodium.
A more general object of the invention is to provide a method for producing sputtering targets with reduced levels of mobile ion contaminants.
Another object of the invention is to provide a method for reducing the concentration of mobile ion contaminants in sputtering targets.
Still another object of the invention is to provide a method for fabricating planar sputtering targets that are usable with a minimum of machining.
A further object is to provide a method of making planar sputtering targets by a combination of low temperature pressing and high temperature vacuum sintering.
These and other objects, features and advantages of the present invention will be apparent as the following detailed description is read in conjunction with the accompanying drawings.